An OPOC engine 10, as disclosed in U.S. Pat. No. 6,170,443, and incorporated herein in its entirety, is an asymmetrical configuration. Such an OPOC engine 10 is shown isometrically in FIG. 1. A first intake piston 12′ is the inner piston in one of the cylinders and a second intake piston 12 is the outer piston in the other cylinder. A first intake piston 12 and a first exhaust piston 14 reciprocate within a first cylinder; and a second intake piston 12′ and a second exhaust piston 14′ reciprocate with a second cylinder (cylinders not shown to facilitate viewing pistons). Exhaust piston 14 and intake piston 12′ couple to a journal (not visible) of crankshaft 20 via pushrods 16 (only one of which is visible). Intake piston 12 and exhaust piston 14′ couple to two journals (not visible) of crankshaft 20 via pullrods 18, with each of intake piston 12 and exhaust piston 14′ having two pullrods 18. Because the pullrods and pushrods sit adjacent to each other, a central axis 22′ of the left cylinder is parallel to, but offset from a central axis 22 of the right cylinder. Thus, the engine is wider than it would otherwise be.
One alternative to overcome the offset cylinders is a forked rod, such as is described in U.S. Pat. No. 1,322,824, invented by F. Royce. By employing a forked rod/blade rod configuration within the engine of FIG. 1, the length of the journal (or crank pin) can be reduced. Also, the cylinders are collinear. The width of the engine can be reduced and the unbalanced forces are reduced. However, a disadvantage of such a configuration is that the piston in one cylinder couples with the crankshaft by a forked rod and the corresponding piston in the opposing cylinder couples with the crankshaft by a blade rod thereby increasing part count for the engine. A system for coupling the rods to the crankshaft is desired which allows common parts to be used in the two cylinder, such as is possible with the configuration shown in FIG. 1, while allowing collinear cylinders, such as that shown U.S. Pat. No. 1,322,824.
However, an issue associated with the crankshaft proposed shown in U.S. Pat. No. 1,322,824 is that the crankshaft is built up from multiple parts to allow assembly. That is, for a conventional four-stroke engine in which two connecting rods are coupled to substantially the same portion of the journal of the crankshaft, the connecting rod is subject to both tension and compression. The forked connecting rod encircles the crankshaft and thus is slipped onto the journal at an end of the journal before the crankshaft is fully assembled. The crankshaft can be assembled by welding, fastening, or press fitting together. There are minimal forces acting on the crankshaft in the axial direction so that such a press fit can be sufficient. However, assembly of the crankshaft must ensure that the components are coaxial. It is desirable to avoid an assembled crankshaft to ensure the desired geometry and to simplify the engine assembly process.